The present invention relates to an insulated pipe joint which is formed with a communication passage at its center and which is intended to be used under conditions of high tensile load including both a high tensile load and a rising temperature. The invention further relates to a method for producing this joint. More particularly, the present invention relates to an insulated pipe joint which is formed with a communication passage and which is used to suspend and hold, in an insulating manner, the electrode pipe of an oil sand well for an electrically heating system and to a method for producing this joint.
Recently, an electrical heating system has been proposed for pumping oil out of oil sand in underground deposits. According to this system, two oil wells, which reach an underground oil sand layer and which may be as deep as several hundred meters, are drilled at a preset spacing. Potentials are applied to electrodes positioned at the bottoms of the wells thereby to supply an electric current therebetween to raise the temperature of the oil sand layer and hence to lower the viscosity of the oil contained therein to a sufficient extent that the oil can be pumped out. Since the electric resistance of an oil sand layer is generally higher than that of the layers over the oil sand layer, the tubular electrodes for the power supply buried in the oil sand layer must be electrically insulated from the oil pumping steel pipes. To accomplish this, an insulated pipe joint is coupled to the steel pipe in the oil sand layer and the tubular power supply electrode is coupled to the insulated pipe joint. With the insulated pipe joint having the construction thus far described, it is always subjected to a tensile load. Moreover, once the temperature of the surrounding layer is raised as a result of the electric current, the temperature of the insulated pipe joint itself is raised making it necessary that the joint retain a high tensile strength even under high temperature conditions. Moreover, since the insulated pipe joint is buried underground as deep as several hundred meters under the conditions of having its lower end coupled to the power supply electrode and its upper end coupled to the steel pipe, it is practically inevitable that the insulated pipe joint will contact or collide with the bore wall during its installation. Since the insulated pipe joint has a considerable total weight so that even slight contact results in a remarkably strong mechanical impact, the insulated pipe joint must have sufficient strength to endure such a mechanical impact.
A great deal of research has heretofore been carried out attempting to provide an insulated pipe joint which can satisfy the above-mentioned requirements. A first of the known pipe joint constructions is a flanged tubular article made of metal having its entire surface coated with an organic resin having excellent heat-resisting characteristics such as polytetrafluoroethylene (for example "Teflon.TM." which is trade name of du Pont). This insulated pipe joint satisfies the tensile strength and the mechanical impact requirements. However, the joint has the defect that the insulating characteristics are insufficient in case the coating is thin and that, even if the insulating characteristics are sufficient at room temperature if the coating is thick, the coating tends to separate due to the intrinsic difference in the coefficients of thermal expansion of the metal and resin because the insulated pipe joint is repeatedly heated to a temperature of 250.degree. to 300.degree. C. then cooled in practical operating conditions.
A second known pipe joint construction uses porcelain material. Since the insulated pipe joint is intrinsically required to be water or oil tight, it is necessary to appropriately provide the connection between the steel pipe, the power supply electrode and the porcelain pipe and between the different porcelain pipes. Generally, this has been done by shrink fitting a metal pipe on the outer circumference of the porcelain pipe and then another metal pipe is attached by welding or screw fitting. With this joint construction, the desired water or oil tight characteristics are attained. However, since stress is concentrated at the leading end portion of the porcelain pipe which has been shrink fitted, there is a defect that the mechanical strength, especially, the resistance to impact of the joint is so low that breakage is liable to take place. Moreover, the strength of the shrink fit joint deteriorates with a rising temperature thereby reducing the tensile strength of the joint.
Yet further, there has been proposed a joint in which flanged portions are formed at both ends of a porcelain pipe which are fastened thereto by metal members with packings sandwiched between the contacting surfaces. With this joint, although the sealing effects are sufficient at room temperature, at high temperatures the sealing effect is insufficient due to the differences in the coefficients of thermal expansion between the porcelain and the metal. In addition, since porcelain essentially lacks mechanical impulse force strength as described hereinbefore, porcelain has a high likelyhood of being broken by mechanical impulse forces which are produced during the installation of the joint. It is thus remarkably difficult to use porcelain as a practical matter.